1. Field of the Invention
The present invention relates to an optical multiplex communication system in which Wavelength Division Multiplexing (WDM) transmission/reception and Optical Code Division Multiplexing (OCDM) transmission reception can coexist.
2. Description of Related Art
In order to increase speed or increase capacity of optical fiber communication, optical multiplex communication technology for transmitting a plurality of channels of optical pulse signals simultaneously on one optical fiber transmission line has been studied. As a means of optical multiplex communication, WDM, to identify a channel by the wavelength of optical pulses constituting an optical pulse signal, and OCDM, to identify a channel by pattern matching of encoded optical pulse signals, have been researched.
First, the configuration and function of an example of an OCDM device (e.g. see N. Wada, et al: “Error-free transmission of 2-channel×2.5 Gbit/s time-spread/wavelength-hop OCDM using fibre Bragg grating with supercontinuum light source”, ECOC' 99, 26-30 Sep. 1999, and Japanese Patent Application Laid-Open No. 2000-209186) will be described with reference to FIG. 1 and FIG. 2A to G. FIG. 1 is a block diagram depicting the time-spread/wavelength-hop OCDM system. FIG. 2A to G are diagrams depicting the time waveform of signals at each location of the time-spread/wavelength-hop OCDM system.
This OCDM system is comprised of a transmission section 40 and a receive section 60 connected to a transmission line 52. The transmission line 52 is an optical fiber. FIG. 1 shows a device assuming 2 channel transmission/reception to prevent explanation from becoming unnecessarily complicated. It is obvious from the description herein below that an OCDM system which allows 3 or more channels of transmission/reception can be implemented in the same way merely by increasing the number of channels.
The transmission section 40 has an encoder 44 of the first channel, and encoder 48 of the second channel and an optical multiplexer 50. The encoder 44 of the first channel encodes an optical pulse signal 43 of the first channel with a code provided as code 1, and outputs it as the encoded optical pulse signal 45 of the first channel. The optical pulse signal 43 of the first channel is output from a signal generator 42 of the first channel. The encoder 48 of the second channel also encodes an optical pulse signal 47 of the second channel with a code provided as code 2, and outputs it as an encoded optical pulse signal 49 of the second channel. The optical pulse signal 47 of the second channel is output from a signal generator 46 of the second channel.
FIGS. 2A and B are diagrams depicting the time waveforms of the optical pulse signals of the first and second channels respectively, and one optical pulse constituting each optical pulse signal is shown respectively as a representative example. An optical pulse constituting the optical pulse signal of the first and second channels includes optical components of different wavelengths λ1, λ2 and λ3. To illustrate this, a rectangle enclosing number 1, 2 or 3, which represents wavelength λ1, λ2 or λ3, is stacked on a same point in time. Here an optical pulse signal, comprised of three different types of wavelengths, will be described. However, the number of types of wavelengths included in an optical pulse is not limited to three, but the same description will be applied to the case when two or four or more types of wavelengths are included.
An optical pulse including optical components with different wavelengths λ1, λ2 and λ3 means that if this optical pulse is separated and arrayed on the wavelength base, this optical pulse is dispersed into optical pulses of which the central wavelengths are λ1, λ2 and λ3. An optical pulse comprised of a single wavelength optical component, which is acquired by dividing the wavelength of an optical pulse including a plurality of optical components, may be called a “chip pulse” herein below.
Hereafter an optical pulse including different wavelength components is shown by stacking up a rectangle enclosing an identification number to indicate the wavelength of that wavelength component on a same point in time. In FIG. 2A to G optical pulses of the second channel are shaded with hatching in order to identify an optical pulse of the first channel and an optical pulse of the second channel.
FIGS. 2C and D show an encoded optical pulse signal 45 of the first channel and an encoded optical pulse signal 49 of the second channel on a time base. As FIG. 2C shows, in the case of the encoded optical pulse signal 45 of the first channel, for example, the optical pulse constituting the optical pulse signal 43 of the first channel is dispersed, by the encoder 44, into optical pulses (chip pulses) having central wavelengths λ1, λ2 and λ3, which are time-spread on the time base. As FIG. 2D shows, the encoded optical pulse signal 49 of the second channel is also dispersed into chip pulses and are time-spread on the time base. The code being set in the encoder of the first channel (code 1) and the code being set in the encoder of the second channel (code 2) are different codes, so the positions of the respective chip pulses of the encoded optical pulse signals of the first and second channels arrayed on the time base are different, as shown in FIGS. 2C and D.
In this way, the encoding performed by the transmission section 40 of the device shown in FIG. 1 is a method of time-spreading the optical pulse on the time base, and encoding by dispersing it into the optical pulses (chip pulses) having central wavelengths λ1, λ2 and λ3 constituting the optical pulse, so this is called “encoding by time-spread/wavelength-hop codes”. In other words, encoding by time-spread/wavelength-hop encoding is performed for the optical pulse signals 43 and 47 of the first and second channels by the encoder 44 of the first channel and the encoder 48 of the second channel respectively.
FIG. 2E shows the optical code division multiplex signal 51 when the encoded optical pulse signal 45 of the first channel and the encoded optical pulse signal 49 of the second channel are multiplexed by the multiplexer 50. The multiplexer 50 plays a part of a multiplexer for multiplexing a plurality of channels of optical signals. As FIG. 2C shows, the chip pulse string constituting the encoded optical pulse signal 45 of the first channel and the chip pulse string constituting the encoded optical pulse signal 49 of the second channel are superimposed on the same time base.
The optical code division multiplex signal 51 propagates the transmission line 52, and is sent to the receive section 60. The receive section 60 has a splitter 62, a decoder 64 of the first channel and a decoder 68 of the second channel. The splitter 62 divides the intensity of the optical code division multiplex signal 51 and supplies one to the decoder 64 of the first channel as a split optical code division multiplex signal 63, and supplies the other to the decoder 68 of the second channel as a split optical code division multiplex signal 67. The decoder 64 of the first channel decodes the split optical code division multiplex signal 63 with a code provided as code 1, which is regenerated and output as an optical pulse signal 65 of the first channel, and is input to the signal receive section 67 of the first channel. The optical pulse signal 65 is recognized as a receive signal of the first channel in the signal receive section 67 of the first channel.
The decoder 68 of the second channel also decodes the split optical code division multiplex signal 67 with a code provided as code 2, which is regenerated and output as the optical pulse signal 69 of the second channel, and is input to the signal receive section 71 of the second channel. The optical pulse signal 69 is recognized as a receive signal of the second channel in the signal receive section 71 of the second channel.
The optical pulse signal regenerated by the decoder of the receive section may be called a “decoded optical pulse signal” herein below.
FIGS. 2F and G show decoded optical pulse signals when the intensity of the optical code division multiplex signal 51 is divided by the splitter 62 of the receive section 60 for each first and second channel, and decoded by the decoder 64 of the first channel and the decoder 68 of the second channel.
The decoded optical pulse signal 65 of the first channel will be described. In FIG. 3F, which shows the light intensity of the first channel on the time base, the chip pulse which comes from the optical pulse signal of the second channel is indicated by a hatched rectangle enclosing a number to identify the wavelength, while the rectangle enclosing a number to identify the wavelength is not hatched for the chip pulse which comes from the optical pulse signal of the first channel.
The chip pulse, which comes from the optical pulse signal of the first channel, is a chip pulse encoded with a code provided as code 1 and generated, so if decoded with the same code provided as code 1, each chip pulse is arranged to be the same position on the time base, canceling the time delay generated during encoding. In other words, an original optical pulse signal is regenerated as an auto-correlation waveform.
In the case of the time waveform of light intensity with respect to the time base of the first channel shown in FIG. 2F, unshaded rectangles enclosing numbers 1, 2 and 3 are stacked at a same point in time. On the other hand, the shaded rectangles enclosing numbers 1, 2 and 3 are distributed at different locations on the time base and appear as a cross-correlation waveform. The shaded rectangle enclosing the number 1, 2 or 3 is a chip pulse which comes from the second channel and is a chip pulse constituting the enclosed optical pulse signal encoded with code 2. In other words, the encoded optical pulse signal components of the chip pulse, which comes from the second channel, are formed as a time-spread cross-correlation waveform again, since encoding and decoding are executed with different codes, and therefore the time delay generated during encoding is not cancelled during decoding.
The time waveform of the light intensity with respect to the time base of the second channel in FIG. 2G has an opposite relationship from above. In other words, the chip pulse which comes from the second channel forms an auto-correlation waveform, and the chip pulse which comes from the first channel forms a cross-correlation waveform. This is because the chip pulse which comes from the second channel is encoded with the code provided as code 2, and decoded with the code provided as code 2.
The optical code division multiplex signal 67, of which intensity was divided, is decoded with the code provided as code 2, so in the case of the chip pulse which comes from the first channel encoded with the code provided as code 1 included in the optical code division multiplex signal 67, the time delay generated during encoding is not cancelled during decoding, and a time-spread cross-correlation waveform is formed again. For the chip pulse which comes from the second channel encoded by the code provided as code 2 included in the optical code division multiplex signal 67, on the other hand, the time delay generated during encoding is cancelled during decoding, and an auto-correlation waveform is formed again.
As described above, the decoded optical pulse signal 65 of the first channel and the decoded optical pulse signal 69 of the second channel are formed as the sum of an auto-correlation waveform and a cross-correlation waveform respectively. As FIGS. 2F and G show, the auto-correlation waveform and the cross-correlation waveform have different peak intensities (peak of the auto-correlation waveform is larger), so if the cross-correlation waveform components are removed by performing threshold judgment, that is judging whether the peak value of the waveform is higher or lower than the threshold value which is set in advance, then only the auto-correlation waveform components can be extracted. If the auto-correlation waveform components can be extracted in each channel, the transmitted information can be received by converting the auto-correlation waveform, which is the respective regenerated optical pulse signal, into an electric signal.
In addition to the time-spread/wavelength-hop method, a method of encoding an optical pulse signal using a single wavelength light may be used for the encoding and decoding method. In the case of this method, an optical pulse constituting the optical pulse signal is separated into chip pulses, and encoding is performed by assigning a phase difference to each chip pulse, and arranging them on the time base (for example, see P. C. Teh, et al: “Demonstration of a Four-Channel WDM/OCDMA System Using 255-Chip 320-Gchip/s Quarternary Phase Coding Gratings” IEEE Photonics Technology Letters, Vol. 14, No. 2, February 2002, pp. 227 to 229). This encoding is sometimes called “encoding by time-spreading”.
As an example of the means of implementing encoding and decoding, Super Structure Fiber Bragg Grating (SSFBG) is known. SSFBG is constructed by arranging a unit fiber Bragg grating (FBG) in series along the wave guiding direction. From each unit FBG, Bragg reflected light having a specific wavelength is generated.
The structure and operation of an encoder based on SSFBG will now be described with reference to FIGS. 3A and B. FIG. 3A is a diagram depicting the configuration of SSFBG which is constructed by a unit FBG1, unit FBG2 and unit FBG3 of which Bragg reflection wavelength is λ1, λ2 and λ3 respectively, arrayed in series along the wave guiding direction. As FIG. 3A shows, when incident light including three types of wavelengths λ1, λ2 and λ3 enters SSFBG, Bragg reflected lights of which wavelength is λ1, λ2 and λ3 are reflected from each unit FBG to the input end. FIG. 3B shows the effective refractive index distribution of the core of the optical fiber where SSFBG is formed.
The refractive index modulation period (may be called “grating pitch”) of the unit FBG1, unit FBG2 and unit FBG3 are Λ1, Λ2 and Λ3 respectively as FIG. 3B shows. Generally the refractive index modulation period Λ and the Bragg reflection wavelength λ have the relationship λ=2nΛ. Here n is an average refractive index of FBG. In other words, the Bragg reflection wavelength λ of the unit FBG is determined by determining the grating pitch Λ of the unit FBG.
If a plurality of unit FBGs having different grating pitches are arranged in series in one optical fiber, a light with a waveform corresponding to the grating pitch (hereafter may be called “Bragg reflected light”) is acquired from each unit FBG. The Bragg reflected light which is reflected from each unit FBG is reflected with a different time delay respectively according to the location where the unit FBG is positioned. Encoding by time-spread/wavelength-hop based on SSFBG uses this phenomena.
Now a configuration example of the encoder using SSFBG will be described with reference to FIG. 4. The decoder has the same configuration, so only the encoder is described here. The encoder in FIG. 4 is comprised of an SSFBG 10 and an optical circulator 18. The SSFBG 10 is further comprised of a unit FBG1, unit FBG2 and unit FBG3.
Optical pulses to be encoded are input from an input port 14 to the SSFBG 10 via the optical circulator 18 as an input light. The SSFBG 10 has the unit FBG1, unit FBG2 and unit FBG3, so the Bragg reflected lights having different wavelengths are reflected from each unit FBG. These Bragg reflected lights are output from an output port 16 via the optical circulator 18 as encoded optical pulses.
An encoder that can implement encoding by time-spread/wavelength-hop, other than the above mentioned SSFBG, is a means constructed by combining AWG (Arrayed-Waveguide Grating) and an optical delay line (e.g. see S. Yegnanarayanan, et al: “An incoherent wavelength hopping/time spreading code-division multiple access system”, ECOC' 99, 26-30 Sep. 1999).
A method of extracting auto-correlation waveform components by separating the auto-correlation waveform components and the cross-correlation waveform components from the optical pulse signal decoded at the receive side, other than the above mentioned method of using threshold judgment, is a method based on a time gate. The method based on a time gate is a method of using a time gate means which adjusts the time so that the cross-correlation waveform component and the auto-correlation waveform component do not overlap, and allows signals to pass only in the time zone when auto-correlation waveform components pass.
As the time gate means, a method based on a time gate using an electron-absorption modulator (EA modulator) is known (e.g. see Naoki Minato, et al: “Transmission design and evaluation of data rate enhanced time-spread/wavelength-hopping optical code division multiplexing using fiber-Bragg-grating” Technical Report IEICE, CS 2003-17, OCS2003-24, PS2003-24, May 2003, pp. 49 to 54). In other words, a time gate is implemented by increasing the transmittance of the EA modulator only in the time zone when the auto-correlation waveform components pass, and decreasing it in the time zone when cross-correlation waveform components pass. Clock signals are used for the control of transmittance of the EA modulator.
Also as the time gate means, a method based on a time gate using an SOA (Semiconductor Optical Amplifier) is known (e.g. see K. Kitayama, et al: “Optical Code Division Multiplexing (OCDM) and Its Applications to Photonic Networks” IEICE Trans. Fundamentals, Vol. E82-A, No. 12, December 1999, pp. 2616 to 2626). According to this method, an optical clock is extracted first from a part of a signal decoded by a mode locked laser diode. Then the decoded signal and the optical clock are input to the SOA, and are synchronized in the SOA to generate a Four Wave Mixing (FWM) effect. And the time gate means is implemented by allowing only optical pulses in the time zone, when SOA is in ON status, to pass the SOA by the Four Wave Mixing effect generated synchronizing the optical clock.
A feature of the optical communication system based on OCDM is that the increase/decrease of the number of channels can be handled flexibly. In the optical communication system based on OCDM, a channel can be added merely by adding the type of codes, only if the size of the ratio of the peak value of the cross-correlation waveform components and the peak value of the auto-correlation waveform components can be assured to be at a level where the cross-correlation waveform components can be removed from the decoded optical pulse signal so as to extract the auto-correlation waveform components. In other words, a new channel can be added merely by adding an encoding section and a decoding section where a new code corresponding to the new channel to be added is set, without changing the composing portion for the channels, other than the channel to be added in the optical communication device.
The transmission/reception based on OCDM has the above mentioned advantageous feature where a plurality of channels of optical pulse signals can be transmitted simultaneously in one optical fiber transmission line.
Therefore if a system, where WDM transmission/reception and OCDM transmission/reception can coexist, can be implemented by attaching the OCDM transmission/reception system to the WDM transmission/reception system in parallel, the number of channels which allow transmission/reception can be further increased. But in order to implement a system where WDM transmission/reception and OCDM transmission/reception can coexist merely by attaching the OCDM transmission/reception to the WDM transmission/reception system in parallel, the wavelength band of a conventional optical multiplex communication system based on WDM must be changed.